In general, when preparing an emulsified product, a nonionic surfactant or an ionic surfactant is used as an emulsifier.
Nevertheless, because the oily substance to be emulsified exhibits various polarities, to obtain a stable emulsified substance, a measure of first obtaining the degree of polarity (the "required HLB") of the oily substance and then selecting a surfactant in conformity with that degree of polarity before using the agent, is taken. As the emulsifier, in many cases a hydrophilic emulsifier with a high HLB value and a lipophilic emulsifier with a low HLB value are combined with one another.
As a lipophilic emulsifier with a high HLB value, e.g., anionic surfactants such as fatty acid soap and alkylsulfuric ester salt; cationic surfactants such as distearyldimethylammonium chloride and stearyltrimethylammonium chloride; and nonionic surfactants having a long polyoxyethylene chain length, e.g., polyoxyethylene alkylether, polyoxyethylene fatty ester and polyoxyethylene sorbitan fatty ester, are used.
Further, as a lipophilic emulsifier with a low HLB value, e.g., a nonionic surfactant with a short polyoxyethylene chain, sorbitan fatty esters and glycerine fatty esters are used.
The required HLB value of an oily substance to be emulsified is obtained by using a nonionic surfactant having an already known HLB value. Very complex means are required for obtaining the required value, e.g., the ratio of the amounts of a surfactant with a high HLB and a surfactant with a low HLB is varied. Further, an emulsifier is selected on the basis of the required HLB thus-obtained, and an emulsified product is prepared using this emulsifier. Nevertheless, a stable emulsified product is seldom obtained in practice, and thus experiments must be repeated on a trial and error basis.
To cope with the above problem, it is disclosed that an emulsifier containing an alkanolamine of oleic acid and an anionic surfactant can emulsify a comparatively wide range of HLB (refer to Japanese Unexamined Patent Publication No. 61-114724), but an emulsified product prepared by using the above surfactant as an emulsifier has a drawback in that it has a high skin irritation effect.
Conversely, an ampholytic surfactant is known to exhibit a low skin irritation, and a large number of emulsified products consisting essentially of an ampholytic surfactant, e.g., a low skin irritation detergent composition, or a shampoo composition with a low irritation of the eyes, etc., have been disclosed (refer to the official gazettes of Japanese Unexamined Patent Publication No. 57-90099 and U.S. Pat. No. 3,950,417).
The ampholytic surfactant disclosed in these official gazettes, however, do not exhibit a strong emulsification of an oily substance with a wide range of required HLB, and furthermore, an emulsion type of O/W or W/O is difficult to control and a stable emulsified product can not be formed.
Accordingly, there has not been obtained an emulsifier displaying a strong emulsification of even an oily substance with a wide range of required HLB, capable of easily controlling the emulsification type, and capable of producing a stable emulsified product having a low skin irritation.